Method and apparatus for producing function potentiometers

ABSTRACT

A method is disclosed for producing conductive plastic potentiometer elements by spraying the conductive plastic on a substrate and then molding to obtain a flush surface.

United States Patent Zablocki 11 1 3,684,998 1 51 Aug. 15,1972

[54] METHOD AND APPARATUS FOR PRODUCING FUNCTION POTENTIOMETERS [72]Inventor: Henry S. Zablocki, 181 Hillside Ave., Nutley, NJ. 071 10 [22]Filed: Jan. 16, 1968 [21] Appl. No.: 698,278

Related US. Application Data [63] Continuation-in-part of Ser. No,530,465, Feb.

28, 1966, Pat. No. 3,371,138, which is a continuation-in-part of Ser.No. 434,180, Feb. 23, 1965, abandoned, which is a continuation-inpart ofSer. No. 95,449, Jan. 25, 1961, abandoned.

1511 1111.111. ..H0lc 5/02 [58] FieldofSearch ..338/160-l62,174,338/176, 211, 328, 330, 307, 314, 311, 308,

Primary Examiner-Lewis H. Myers Assistant Examiner-A. T. GrimleyAttorney-Leonard H. King [57] ABSTRACT A method is disclosed forproducing conductive plastic potentiometer elements by spraying theconductive plastic on a substrate and then molding to obtain a flushsurface.

3 Claims, 14 Drawing Figures PATENTEUMJB is me 2 684 99a SHEET 1 [IF 4FIG. 4

. INVENTOR HENRY S. ZABLOCKI {km/vi H.

HIP

PATENTEDAHB 15 1912 3,5 4,99

SHEET 2 or 4 HENRY S. ZABLOCKI i l MM H.

Pmmmws 15 m2 3.684.998

SHEET 3 OF 4 I90 I I T I a I INVENTOR HENRY 3.. ZABLOCKI PATENTEDAU: 15I972 3584.998

snwunm FIG. 84

INVEN TOR. HENRY S. ZABLOCKl ATTORNEY METHOD AND APPARATUS FOR PRODUCINGFUNCTION POTENTIOMETERS This application is a continuation-impart of mycopending application Ser. No. 530,465, filed Feb. 28, 1966 Now U.S.Pat. No. 3,371,138 which in turn is a continuation-in-part of myapplication Ser. No. 434,180, filed Feb. 23, 1.965 and now abandoned,and which in turn is a continuation-in-part of my earlier filedapplication Ser. No. 95,449, filed Jan. 25, 1961, and now abandoned.

This invention relates to methods, and apparatus employing such methods,for making precision function potentiometers of the conductivecomposition type. By precision potentiometers, it is intended toencompass that class of potentiometer characterized by a high conformitywith the desired function, that is, a deviation from the function ofless than plus or minus 2 percent. Precision potentiometers are oftenrequired to have a conformity as close as 0.03 percent to function.

One conventional method of making potentiometers of the carboncomposition type is by coating an insulator plastic or ceramic base byspraying with a composition containing conductive carbon particlesdispersed in a resinous binder. Most commonly, the resulting resistanceelement is in the form of an annular path which is formed by spraying adispersion of the composition through a mask. Conventionally, an axiallylocated shaft is rotated to move a wiper making contact to theresistance element. Suitable takeoff means, common to the art, areemployed for providing electrical connection to the potentiometerterminals.

In order to provide a function potentiometer, e.g., one whose outputvoltage varies in a nonlinear fashion with variation of the shaft angle,the resistance of the coating per unit length must be made to vary inaccordance with the desired function. Conventionally, this is done byjudiciously varying the width of the coating so as to provide a path ofgreater of lesser conductivity per unit length. This technique hasproved adequate for producing low cost variable resistance devices foruse as gain and volume control means, such as commonly found in radioreceivers and the like. In applications of the latter class, thevariable resistance device is adjusted to provide a desired outputlevel. Once the desired level is attained, the setting is not normallydisturbed. The degree of precision of the potentiometer is not material;dial settings are used merely as a guide in such applications. However,this procedure is not suitable for producing precision potentiometers.Precision potentiometers are often employed in feedback type servosystems, and in analog computers as position sensing means and as abalancing control means where continuous precise and sensitiveadjustment in accordance with a signal is required. Obviously, for suchapplications a high degree of precision and accuracy is required.Briefly stated, one aspect of this invention comprises the forming of aresistance track by means of a collimated source of material and movingthe substrate and said source, one relative to the other, whileautomatically varying the velocity in relation to a desired function ina manner to be more fully described hereinafter. Another aspect of thisinvention relates to an apparatus for carrying out the said process.

Accordingly, it is an object of this invention to provide a process foreconomically producing composition type function potentiometers of theprecision class.

A further object of this invention is to provide an apparatus fordepositing a coating on a substrate in a controlled nonlinear fashion.

It is an object of this invention to provide an apparatus forautomatically fonning function potentiometer elements.

It is still a further object of this invention to provide an apparatusfor accurately forming potentiometer elements.

A particular object of this invention is to provide an improved servocontrolled potentiometer-forming apparatus.

A further object is to provide an apparatus for forming a multilayersprayed function potentiometer element.

A particular object of this invention is to provide means for monitoringthe actual function being generated by the potentiometer-formingapparatus.

Still other objects and features of this invention will, in part, becomeobvious and will, in part, be pointed out with particularity as thefollowing description proceeds when taken in conjunction with theaccompanying drawing.

In the drawing:

FIG. 1 shows schematically one embodiment of the apparatus of thisinvention;

FIG. 2 is a plan view of a potentiometer produced by the teachings ofthis invention;

FIG. 3 is a schematic showing of another embodiment of the claimedapparatus;

FIG. 4 shows schematically still another embodiment of the apparatus;

FIG. 5 is a plan view of a sampling switch used in the embodiment ofFIG. 4;

FIG. 6 is a plan view of a rectilinear potentiometer element in theprocess of being coated;

FIG. 7 is a plan view of a flat mold force suitable for molding anarcuate variable resistance device with an apertured masking memberaffixed to the mold force. The portion of the mold member exposedthrough the mask aperture is shown coated with a resistive composition;

FIG. 8 shows the mold force of FIG. 7 with another masking deviceemployed in a subsequent operation;

FIG. 9 shows in a vertical section, a mold assembly producing the moldedresistance element of FIG. 10;

FIG. 10 is a plan view of a molded resistance element produced by theapparatus of this invention;

FIG. 11 is a cross sectional view taken along lines ll-l1 ofFlG. 10.;

FIG. 12 shows in perspective an improved collimator of this invention;

FIG. 13 shows schematically a coating apparatus having meansincorporated for preventing stalling; and

FIG. 14 shows schematically an apparatus for providing continuousmonitoring of the operation.

The conductivity of a track of uniform material, theoretically at least,is directly proportional to the thickness of the track. This isexpressed by the classic formula for conductance of a uniform bar ofmaterial C wt/pl where w, t, l and p are the width, thickness, lengthand specific resistivity, respectively.

Where an insulator substrate is coated with a resistive track of auniform material, the conductivity is directly proportional to thethickness of the layer of material applied. Where the coating is appliedby paint ing, some finite thickness is required before conductivity isestablished. However, for practical purposes this minumum thickness maybe disregarded.

Consider now the manufacture of a potentiometer whose output voltage, E,is proportional to a function of the angular position of the shaft andthus of the angular position of the wiper on the resistive track,expressed thus:

Eaf(6) Kirchoffs Law states that the total voltage drop in a seriescircuit is equal to the sum of the individual voltage drops. Expressedin differential terms,

where r expresses the resistance of a differential length of track as afunction of 0. Differentiating,

(dE/d0)a r (0) and since E=f(0) (dE/d0)ar(0)f (0) Thus, given acollimated spray of resistive material, and a surface moving at a givenvelocity relative to an orifice from which the spray is emanating, theamount of material deposited is inversely proportional to the givenvelocity and accordingly the resistance of the track is directlyproportional to the given velocity, and the incremental resistance islikewise directly proportional to the velocity of the mold or substrateupon which the resistive material is being deposited.

Thus, r(0) a v(6) af (0) Therefore, the resistance from the start of theresulting track to any point thereon is defined by the followingequation:

This relationship provides a convenient means for monitoring theaccuracy of the process.

Referring now to FIG. 1, there is shown a motor 12 coupled to shaft 14upon which is keyed a potentiometer insulator base 16. A reinforcedplastic, such as glass fiber cloth impregnated with epoxy resin, issuitable for this purpose. A thin sheet metal mask 16a provided with aslot, conforming with the shape of the resistance track, is secured tothe face of the base 16 by means of pins inserted into holes therein.The mask also shields the area between terminals so that it is notcoated. As motor 12 rotates shaft 14 at a moderate speed, say, rpm,member 16 rotates past orifice 18 in mask 20. This orifice,approximately 5 wide, permits conductive carbon dispersion 21 from spraygun 22, to be deposited on substrate 16. It will be appreciated that thefaster the motor rotates, the less material will be deposited during agiven pass. By depositing a plurality of thin layers of resistancematerial, a more uniform and smooth coating is obtained than bydepositing one thick layer as would result if the base were rotated,for' example, but once during the coating cycle.

A master control signal is obtained from a constant voltage power source19 which energizes master potentiometer 23. Potentiometer 23 is providedwith a function proportional to the derivative of the function desiredin the potentiometer under production. it should have a higher degree ofconformity with function than the potentiometer element underproduction.

As shaft 14 rotates substrate 16, the potential of wiper 25 will vary inaccordance with the derivative of the desired function and provide amaster control signal to amplifier 32. Motor 12, under the control ofamplifier 32, is forced to change speed in response to the mastercontrol signal. A feedback loop is provided which includes a DC.tachometer 34 coupled in tandem with shaft 14 to servo motor 12. Theoutput of the DC. tachometer 34 is arranged so that the voltage signalis of opposite polarity with respect to the master control signal. Thetwo signals are then compared in amplifier 32 and the difference voltageamplified to drive servo motor 12 at a velocity which will minimize theinput signal to the amplifier; or, stating this another way, the servomotor 12 will tend to rotate at a velocity proportional to the voltageobtained from potentiometer 23.

The completed potentiometer element is shown in FIG. 2. It is composedof the insulator base 16, resistive track 40, terminals 43a, 43b and436. Terminal 43c is connected to a precious metal takeoff ring 44. Arotatable wiper assembly 45 (shown diagrammatically) makes connectionfrom the resistive track to the takeofi ring.

One method of making electrically conductive connections between theresistive track and the terminal is by the use of an electricallyconductive lacquer.

This disclosure of a simple embodiment is merely for the purpose ofaiding the understanding of the apparatus and method of making suchdevices and it is to be understood that more elaborate mechanicalconfiguration and construction in keeping with the state of the art maybe employed.

It will be noted that the potentiometer drawing shows the angle a andangle B the angle a being the angle of rotation covered by a wipertraversing the resistance element between terminals and the angle Bcorresponding to 360 oz.

Since the completed potentiometer is provided with a pair of endterminals 43a, 43b, it is necessary that the coating be deposited sothat the terminals occur at the proper point on the resistance track.This is accomplished by phasing the substrate in relation to the shaftand the master potentiometer. This is readily accomplished by providinga keyway 17 in base 16 which engages a mating key 17a on shaft 14.

Coupled to shaft 14 so as to rotate with it, there is provided a cam 31.This cam is so shaped that as the end of the track 10 passes the spray,switch 39 is triggered, reversing the leads to the master potentiometer23 from the power supply 19. By thus reversing the polarity of powersupply, the servo motor 12 is caused to reverse and rotate the element16 being coated in the opposite direction. This arrangement has theadvantage of washing out any minor inaccuracy resulting from normal timelags in system response.

In FIG. 3, an alternate switching arrangement is shown wherein uponactuation of switch 42 by the cam 31, the output of master potentiometer23 is disconnected from the amplifier 32 and voltage source issufficiently high to cause the shaft 14 to rotate through the angle B ata high speed. When the angle has been traversed, the cam no longerengages switch 42 and control is restored to the master potentiometer.

In the embodiment of FIG. 4, master potentiometer 23 is replaced by aseries of individually adjustable voltage dividers and a sampling switch26. A constant voltage power supply 19 energizes the adjustable voltagedividers 24. The adjustable taps of the potentiometers 24 are set toprovide a voltage proportional to the derivative of the function desiredat a particular point on the potentiometer. Sampling switch 26 isprovided with wipers 28a and 28b, coupled to shaft 14. Thus the wipersrotate in synchronism, and phase, with the potentiometer base 16. Wipers28a and 28b sequentially contact switch segments 30 (FIG. 5), the latterbeing connected to respective ones of voltage dividers 24. It will beappreciated that switch 26 serves as a sampling switch to pick out thevoltage proportional to the velocity desired over a particular segmentof the track. This voltage is then fed into servo amplifier 32. Afeedback loop is provided which includes a DC. tachometer 34 having itsoutput fed (in bucking relationship to the command signal) to servomotor 12. The output of the DC. tachometer 34 is compared with thecommand signal voltage from the sampled potentiometer 24 and thedifference voltage is amplified to drive servo motor 12 at a velocitywhich will minimize the input signal to the amplifier; or, stating thisanother way, the servo system will tend to rotate at a velocityproportional to the voltages on the switch.

Contacts 30 are arranged in two concentric rows with a staggeredarrangement shown in FIG. 5. Two wipers 28a and 28b are employed intandem. The two wipers are isolated from each other by a pair of seriesconnected 1,000 ohm resistors 36a, 36b. The voltage output of the wipersis taken from the junction of the two resistors and fed to takeoff ring48 which, in turn, is contacted by wiper 50. As the wipers traverse thecontacts, they alternately establish connection with the next successivecontact before breaking contact with the preceding contact. At oneinstant, when contact is made simultaneously by both wipers to the twoswitch segments, the output voltage will be the average of the twopotentials. It is desireable to restrict the overlap to the minimum soas to prevent the average value from significantly changing the curve.Minor averaging serves to smooth the function curve. This arrangementavoids interruption of the circuit between contact of successivecontacts and prevents the servo motor 12 from stopping between segments.

The preferred sampling switch design employing staggered contacts is notavailable as a standard component and accordingly the conventional typewith radially aligned contacts may be employed in combination with apair of wipers (corresponding to wipers 28a and 28b) which arecircumferentially offset from the longitudinal axis of their supportingbrush block, the longitudinal axis being a radial line extending fromthe center of rotation. While as pointed out above, the slight overlapbetween contact points is intended to prevent motor 12 from stallingbetween segments, slight inaccuracies in manufacture of switches,particularly mass produced switches, wear, tweaking of the wipers whichare generally fine precious metal wires, and other relatedirregularities can cause occasional malfunctioning of the equipment. Forinstance, simultaneous entry of wipers 28a and 28b onto insulatorsegments of sampling switch 26 can cause the motor 12 to stall wherebyrotation of mold l6 ceases. The embodiments shown in FIGS. 1, 3 and 4provide no means for restarting rotation of the mold. The result is alarge build-up of conductive material at the corresponding point on theresistive path which is being coated by the apparatus. In order toovercome this problem there is provided the system shown in FIG. 13which adds a constant speed motor 12. Both motors l2 and 12a are coupledto the sampling switch 26 through a mechanical differential 13 whichalgebraically adds the velocities of the two motors. Battery 19 of theprior embodiment is replaced by a center tapped battery 19a and theresistors 24 connected across the battery which serves to rotate motor12 in either of two directions responsive to the information stored inresistors 24. The characteristics of motor 12 are such that its maximumvelocity is less than that of motor 12a. Accordingly the net velocity ofsampling switch 26 never reaches zero. Thereby the problem of stallingof the motor is eliminated.

The servo system should have a response time of the order of 40milliseconds time constant and should have sufficient response toovercome inertia of the system. The design criteria for accomplishingthis are well known to those engaged in the art of designing servosystems.

In carrying out this invention a collimator is employed for forming ashaped coating beam. The term collimator is used herein in a sensesomewhat different from, but related to, its use in optics. It describesa device which serves to define the direction and limit the area anddeviation from parallelism of a portion of the stream of materialissuing from a spray gun. In this respect the device acts much as anoptical collimator selects and defines the path of a portion of a streamof light. As described herein, a suitably shaped and positioned orificeacts in cooperation with a spray nozzle to so define a stream of spraymaterial. In FIG. 1, there is shown the outline of the collimated beamas applied to a potentiometer of the rotational type. The side edges ofthe beam are radially aligned with respect to the axis of rotation; thetop and bottom are arcuate. The beam is restricted to an angle of trackcoverage between 1 and 10 It is important that the spray beam becentered with respect to the collimator opening to obtain a uniformspray pattern.

With respect to the dimensions of the collimator opening a width of 0.01inch is not impractical, though it is preferred to use one, two or threetimes as wide. There is some tendency for the material being sprayed topile up around the orifice and then to collapse and pass through theorifice in a lump. It should be understood that many passes are requiredfor coating a mold, or substrate 16, and that this operation may takefrom about 10 minutes to about an hour. This requires that thecollimators be cleaned quire frequently during operation. To minimizethis difficulty, it is preferred to employ the double collimator 20a and20b, shown in FIG. 12. This device is claimed in the copendingapplication of Frank S. Rudo, now US. Pat. No. 3,147,142,

issued Sept. 1, 1964. In this device are two orifices 18a, 18b, ofsimilar shape in line, the one closer to the gun orifice 18a beingslightly larger than the one closer to the mold (orifice 18b). Orifice18a collects most of the buildup and if a lump of it goes through theopening it hits the second collimator 20b and does not reach the mold.This device substantially reduces the number of times the collimator hasto be replaced or cleaned during the coating operation. The same type ofdouble collimator may be used for coating base 16.

On the other hand, FIG. 6 shows the outline of a beam 52 for forming arectilinear track 51. In this case, the beam has parallel edges whichare at right angles to the track. The width of the beam should becomparable to that used for rotary potentiometers.

A preferred method of making the potentiometer device is shown in FIGS.7-11. Mold force 62 is mounted on shaft 14 in place of substrate 16 andcovered with a mask 63. The mask 63 is formed of thin sheet metal and isprovided with an arcuate slot 64. The mask is secured to the mold forceby means of pins 63a which seat in bores in the mold force. A resistivecomposition 21 is then sprayed over the surface of the mold forceexposed through the slot 64 to form a resistance track 66. It will benoted that the mask forms a gap between the ends of resistance track 66.Mask 63 is removed and a mask 67 positioned over the resistance track66, as shown in FIG. 8. Cutouts 69 and 70 outline terminal members whichare formed by spraying the exposed surface with a highly conductivematerial such as a dispersion of finely divided silver flake and alcoholwith about percent by weight of silver of a resin binder. The mask isremoved and the mold force inserted into a collar 71, as in FIG. 9.Insulator plastic 72 is added and an upper mold force 73 is insertedinto the collar. Sufficient heat and pressure suitable for theparticular resin employed is then applied to the mold to form theresistance shown in FIGS. and 11. Thus, there has been provided aninsulator plastic base 75 having molded thereto a composition functionresistance track 66 and terminal members 76 and 77. A conventional wipermeans 80, shaft 81 and takeoff 82 are shown. The advantage of thismethod is that a flat surface is presented to the wiper 80 although thethickness of the resistance track varies in accordance with therequirements of the desired function.

In the day to day operation of the coating machines describedheretofore, it has been found that occasionally the scheduling resistorsare mis-set, or due to change in the resistance of for other reasons,the apparatus otherwise mis-functions so that the correct instantaneousspeeds of rotation of the mold face or substrate are not maintained. Theimprovement shown in FIG. 14 makes it possible for the operator tomonitor the integral of the actual speed of rotation of the mold face.The integral of the actual speed is directly proportional to the desiredfunction output of the completed potentiometer. It has been earlierpointed out that the resistance from the start of the track to any pointis defined by the following equation 1m L We) dof(0) This relationshipmay be employed for monitoring the accuracy of the process. Anintegrating amplifier 90,

such as is well known in the analog computer art, is connected to theoutput of tachometer 34, which has an output directly proportional tovelocity. The output of the integrator is then a direct measure of theresistance of the track. If the integrator is reset at the end of eachrevolution, then the instantaneous output is a direct measure of thefunction output of the potentiometer. Any malfunctioning of the systemor incorrect programming will show up as an output from the integratorthat is different from the desired output. As shown in FIG. 14, theoutput from tachometer 34 is fed through resistor 91 to amplifier 90. Acapacitor 92 is connected between the input and the output of theamplifier. The choice of constants for the resistor and capacitor is inaccordance with conventional practice for an integrator. Cam 93 on shaft14 closes switch 95 to discharge the capacitor whenever the spray ofconductive material is directed on the closed portion 96 of mask 16A.The output from the integrator can be observed at oscilloscope 97 andthe shape of the trace compared with a template 99 or a reference curvedrawn on the oscilloscope face.

As brought out by FIG. 6, the method of making a rectilinearpotentiometer is the same as employed for making a rotational typepotentiometer.

If desired, a raised track may be made by employing a mold provided witha track receiving recess as is taught, for example, in US. Pat. No.2,700,719.

In this instance, the resistance composition is sprayed into the recess.The use of a mask is not necessary as it is a simple matter to scrapethe surface of the mold adjacent to the recess to remove any excessmaterial deposited thereon. This is not to say that the collimated spraysource be dispensed with.

The use of a depression is preferred because in the later molding step,the track material is confined against sidewise movement by the walls ofthe depression. The gap area between the terminals may be protected bytape or by a small mask or the area may be cleaned out after applicationof the track material.

It is to be understood that while in the foregoing description of thepreferred embodiment, reference is made to employment of a stationaryspray source and a moving substrate, the substrate to be coated may bemaintained in a fixed position and the spray source moved.

A preferred system has thus been described where control data issupplied as an electrical signal, feedback data is provided as anelectrical signal from a tachometer, and actuation is through anelectronic servo system. One skilled in control system engineering willreadily devise mechanical, hydraulic or pneumatic control systemsanswering similar functions. Thus, data may be provided in the form ofcams which control sources of mechanical, hydraulic or pneumaticpotential. Other velocity sensitive devices than tachometers, such, forinstance, as governors or turbines may be used. Hydraulic or pneumaticservos may form part of the control system. The electrical systemalready described is preferred for its simplicity and reliability.

It is to be appreciated that the spray gun may have as an integralmember collimating means to produce a beam of the desired pattern.

There has been disclosed heretofore the best em bodiment of theinvention presently contemplated and it is to be understood that variouschanges and modifi' cations may be made by those skilled in the artwithout departing from the spirit of the invention.

What is claimed is:

l. A potentiometer of the class described integrally comolded andcomprising a nonflexible electrically insulating plastic base having anuninterrupted planar surface, a wafer thin spray coating of particlesdeposited on said surface of the base forming a variable resistanceconductive plastic track varying in its depth dimension in proportion tothe resistance characteristic at given points on said track, said trackbeing flush with said surface of the base and the particles of saidcoating being pressed into comingling association with respect to thesurface particles of said surface of the base for receiving a movablewiper thereon, and conductive plastic terminals in electrical connectionwith the underside of said track and flush with said surface of the baseoutside of the track width.

2. A potentiometer of the class described integrally molded andcomprising a nonflexible electrically insulating plastic base having anuninterrupted planar surface, a wafer thin spray coating of particlesdeposited on said surface of the base forming a variable resistanceconductive plastic track of uniform density across its width and varyingin its depth dimensions along the length of said track between maximumand minimum values in proportion to the resistance characteristic atgiven points on said track, said track being flush with said surface ofthe base and containing a plastic insulating bridge flush with saidsurface of the base and the particles of said coating being pressed intocomingling association with respect to the surface particles of saidsurface of the base for receiving a movable wiper thereon, andconductive plastic terminals in electrical connection with the undersideof said track and flush with said surface of the base outside of thetrack width.

3. A potentiometer of the class described integrally comolded andcomprising nonflexible plastic electrically insulating base having anuninterrupted continuous outside surface mountable for rotation about anaxis which is angularly disposed with respect to the surface, a waferthin spray coating of particles deposited on said surface of the baseforming a variable resistance conductive plastic track varying in itsdepth dimension in proportion to the resistance characteristic at givenpoints on said track, said track being flush with said surface of thebase and the particles of said coating being pressed into cominglingassociations with respect to the surface particles of said surface ofthe base for receiving a movable wiper thereon, and conductive plasticterminals in electrical contact with the underside of said track andflush with said surface of the base outside of the track width.

1. A potentiometer of the class described integrally comolded andcomprising a nonflexible electrically insulating plastic base having anuninterrupted planar surface, a wafer thin spray coating of particlesdeposited on said surface of the base forming a variable resistanceconductive plastic track varying in its depth dimension in proportion tothe resistance characteristic at given points on said track, said trackbeing flush with said surface of the base and the particles of saidcoating being pressed Into comingling association with respect to thesurface particles of said surface of the base for receiving a movablewiper thereon, and conductive plastic terminals in electrical connectionwith the underside of said track and flush with said surface of the baseoutside of the track width.
 2. A potentiometer of the class describedintegrally molded and comprising a nonflexible electrically insulatingplastic base having an uninterrupted planar surface, a wafer thin spraycoating of particles deposited on said surface of the base forming avariable resistance conductive plastic track of uniform density acrossits width and varying in its depth dimensions along the length of saidtrack between maximum and minimum values in proportion to the resistancecharacteristic at given points on said track, said track being flushwith said surface of the base and containing a plastic insulating bridgeflush with said surface of the base and the particles of said coatingbeing pressed into comingling association with respect to the surfaceparticles of said surface of the base for receiving a movable wiperthereon, and conductive plastic terminals in electrical connection withthe underside of said track and flush with said surface of the baseoutside of the track width.
 3. A potentiometer of the class describedintegrally comolded and comprising nonflexible plastic electricallyinsulating base having an uninterrupted continuous outside surfacemountable for rotation about an axis which is angularly disposed withrespect to the surface, a wafer thin spray coating of particlesdeposited on said surface of the base forming a variable resistanceconductive plastic track varying in its depth dimension in proportion tothe resistance characteristic at given points on said track, said trackbeing flush with said surface of the base and the particles of saidcoating being pressed into comingling associations with respect to thesurface particles of said surface of the base for receiving a movablewiper thereon, and conductive plastic terminals in electrical contactwith the underside of said track and flush with said surface of the baseoutside of the track width.